This invention relates to the coating of metal substrates with nickel as, for example, is required in the preparation of ceramic carriers employed in the packaging of semiconductor chips.
The increased performance and circuit/bit densities of today's semiconductor chips require corresponding technological advancements in chip packaging. Since the introduction of the leadless ceramic chip carrier, the chip carrier packaging concept has seen increasing use. Ceramic chip carriers typically make use of alumina-based substrates and have discrete areas of multi-layer metallization bonded to the ceramic substrate. These areas of metallization comprise in sequence (a) a base metallization layer bonded to the ceramic substrate, (b) a layer of nickel bonded to the initial, or base, layer and (c) a layer of gold bonded to the nickel layer. As a practical matter tungsten is the metal of choice for the base metallization, although the use of molybdenum has been reported by one manufacturer for internal consumption.
In the typical fabrication of multi-layer ceramic substrates, alumina powder is mixed with glass frit and organic chemicals to form a slurry. This slurry is cast into sheets having a controlled thickness, which sheets are then blanked into various sizes and shapes, and via holes are punched. These green sheets are then screen printed with a metal such as tungsten or molybdenum to form the base metallization. These metallized green sheets are stacked, and laminated together, followed by cofiring, i.e., sintering, in hydrogen or a hydrogen-nitrogen mixture with the heating schedule usually peaking at 1550.degree. C.-1650.degree. C. Thereafter, these sintered substrates are processed to apply nickel metallization over the exposed discrete areas of sintered tungsten. This is followed in turn by gold metallization of the nickel surfaces. Actual compositions of the slurry and specifics of the processing can be expected to vary from manufacturer to manufacturer.
The tungsten metallization is usually about 10 microns thick and is very porous. The nickel layer applied thereto is typically about 2-5 microns thick, this layer being applied by either electrolytic or electroless nickel plating. The nickel functions both to aid in wire bonding and to provide a better thermal expansion match between the tungsten and gold layers on opposite sides thereof. Furthermore, the nickel layer functions as a surface to which a ceramic or metal lid may be attached. The thin layer of gold, typically about 1-2 microns thick, is applied to accommodate die attachment, wire bonding and sealing. It is important that the nickel and the gold metallizations contain as few pores as possible so that heremeticity can be achieved.
The sintered multi-layer ceramic bodies provided with the discrete areas of multi-layer metallization are subsequently subjected to brazing, chip joining and capping operations.
A high purity nickel deposit generally can be obtained by electrolytic plating. It is well known, however, that this process has several major drawbacks, including the following: (a) because of the need for an externally applied electrical current, it is often difficult to plate articles with complex shapes and circuitry; and (b) for the same reason, the nickel coating is generally very nonuniform, being thicker in well-exposed areas and substantially thinner at corners.
Because of these and other disadvantages of the electrolytic mode of plating, nickel metallization of ceramic chip carriers increasingly is carried out by electroless plating. This latter method can plate articles regardless of complexity of shape or circuitry with a relatively uniform coating thickness. This result is due to the fact that the tungsten surface is subjected to pretreatment, rendering the surface catalytic to the nickel deposition, and each unit area should be equally catalytic. Additionally, metals deposited electrolessly plate well into holes and around corners.
Unlike electrolytically deposited nickel, however, electrolessly deposited nickel typically contains a high level, usually in excess of 1 wt %, of boron or phosphorus. These impurities have been associated with the introduction of high internal stress in the nickel coatings. U.S. Pat. No. 4,407,860, issued to Fleming et al., discusses this problem and proposes a composition for the base metallization together with a post-coating treatment that purportedly provides stress-free pure nickel metallization.
Tungsten, as screen-printed and sintered on ceramic chip carriers, has not been found to be catalytic to the deposition of nickel thereon. Rendering of the discrete areas of tungsten catalytic is generally accomplished by depositing a material, e.g., palladium, on the tungsten surfaces to "activate" these substrate surfaces.
Electroless nickel plating compositions essentially consist of a nickel salt, a complexing agent, buffers, and a reducing agent. Nickel compositions which use as a reducing agent a hypophosphite or a borohydride or derivatives thereof do not allow the deposition of nickel on tungsten directly, i.e., without the use of surface activators. It is as a result of using such compositions that boron or phosphorus is embodied in the nickel as referred to hereinabove. Furthermore, while boron-containing reducing agents such as dimethylaminoborane (DMAB) are suitable when electrolessly plating nickel onto many substrates, they are relatively ineffective for plating nickel onto certain refractory metals, such as tungsten and molybdenum.
Certain prior art electroless plating compositions incorporate hydrazine compounds as the reducing agent in combination with ingredients other than those contained in the bath of the present invention. For example, Levy discloses a nickel-plating composition containing hydrazine as the reductant in Electrochemical Technology, Vol. 1, No. 1-2, 1963, pp. 38-42. Also, Gershov et al. discusses hydrazine-containing nickel-plating compositions in Russian Engineering Journal, Vol. 53, No. 10, pp. 73-74; and Vol. 58, No. 5, pp. 37-38. Yet another nickel-plating composition containing hydrazine is disclosed by Dini et al. in Plating, Vol. 54, Apr. 1, 1967, pp. 385-390. Furthermore, U.S. Pat. No. 3,915,716, issued to Haack, discloses a chemical nickel plating bath containing hydrazine or its salts. However, several important components may not be present in these prior art compositions. For example, they may not contain, in addition to the hydrazine, a complexing agent to complex free metal ions in the composition. Furthermore, they may not contain, in combination with hydrazine and the complexing agent, monoethanolamine, which can adjust the pH of the composition while also complexing the free metal ions. The exclusion of such components invariably results in at least one of several disadvantages when using the composition. For example, their use requires activation of the substrate, generally preceded by a series of rigorous cleaning steps, when plating on certain refractory metals, such as tungsten. Furthermore, some of these prior art baths are generally unstable under temperature conditions necessary for plating nickel directly onto nonactivated substrates of this type. Also, some of the prior art compositions require high organic amine-to-water ratios which, in addition to resulting in cost and safety concerns, do not allow nickel to be plated directly on tungsten or molybdenum substrates without prior activation of the substrates.
It is therefore an object of the present invention to provide a stable electroless nickel plating composition which allows nickel to be plated directly upon metal surfaces without prior activation of the surfaces.
It is yet another object of the present invention to provide a reliable and practical method of applying nickel directly to a nonactivated tungsten surface.